In Genesis, the floodwaters began to recede after 150 days. Genesis 8:3 says that the waters steadily receded at the end of the 150 days, after the waters had prevailed over the earth.
This number belongs to the flood’s chronology and is separate from the forty days and forty nights of rain. Genesis also gives other time markers, including the date when the Ark came to rest on the mountains of Ararat and the later stages of waiting before Noah left the vessel.
Chronological details in Genesis can seem difficult because the narrative reports overlapping periods: rain, prevailing waters, receding waters, and Noah’s continued time inside the Ark. The answer is therefore not forty days, even though forty is the best-known number associated with the Flood. It specifically refers to the period before the waters began their retreat.